In general, an organic electroluminescent device (hereinafter, referred to as organic EL device) is constructed of a light-emitting layer and a pair of counter electrodes interposing the light-emitting layer therebetween in its simplest structure. That is, the organic EL device utilizes the following phenomenon. When an electric field is applied between both electrodes, electrons are injected from a cathode and holes are injected from an anode. Those electrons and holes recombine with each other in the light-emitting layer to emit light.
An organic EL device using an organic thin film has started to be developed in recent years. In particular, in order to enhance luminous efficiency, there has been developed a device which is optimized for the kind of electrodes for the purpose of improving the efficiency of injection of carriers from the electrodes, and includes a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of an 8-hydroxyquinoline aluminum complex (Alq3) as thin films between the electrodes, leading to a great improvement in luminous efficiency as compared to a conventional device using a single crystal of anthracene or the like. Thus, studies have been made on the practical application of the device to a high-performance flat panel having features such as self-light emission and rapid response.
Further, studies have also been made on using phosphorescent light rather than fluorescent light as an attempt to raise the luminous efficiency of the device. Numerous devices such as the above-mentioned device including a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of Alq3 utilize fluorescent light emission. However, by using phosphorescent light emission, that is, by utilizing light emission from a triplet excited state, the efficiency is expected to be improved by about three to four times as compared to a conventional device using fluorescent light (singlet). For this purpose, studies have been made on using a coumarin derivative and a benzophenone derivative in a light-emitting layer, but extremely low luminance has only been provided. Further, studies have been made on using a europium complex as an attempt to utilize a triplet state, but highly efficient light emission has not been attained. In recent years, as described in Patent Literature 1, many pieces of research centered on an organic metal complex such as an iridium complex have been made for the purpose of attaining the high efficiency and long life of light emission.